Chain-Length-Dependent Termination in n-Butyl Methacrylate and tert-Butyl Methacrylate Bulk Homopolymerizations Studied via SP-PLP-ESR

The chain-length dependence of the termination rate coefficient, k(t), of bulk homopolymerizations of n-butyl methacrylate (n-BMA) and tert-butyl methacrylate (t-BMA) at ambient pressure and temperatures between -30 and 60 degrees C has been studied via the single pulse-pulsed laser polymerization-electron spin resonance (SP-PLP-ESR) technique. The decay of radical concentration, c(R), after laser SP initiation is monitored with a high time resolution of microseconds by ESR spectroscopy. Radical chain length, i, increases linearly with time t after applying the laser pulse. The experimental k(t)(i,i) values refer to rate coefficients for termination of two radicals of identical chain length i. The variation of k(t)(i,i) with chain length is adequately represented via the composite model proposed by Smith et al., in which two power-law expressions, k(t)(i-i) proportional to i(-alpha), are contained with the exponents alpha(s) and alpha(l) referring to short-chain and long-chain radicals, respectively. The transition between the two regimes occurs at the crossover chain length, i(c). The rate coefficients extrapolated for termination of two radicals of chain length unity. k(t)(l,l), are almost identical for n-BMA and t-BMA with an activation energy of E-A(k(t)(l,l)) approximate to 10 kJ mol(-1). The alpha(s) values are close to each other 0.65 +/- 0.10 (n-BMA) and 0.56 +/- 0.10 (t-BMA). Both alpha(l) values are found to be 0.20 +/- 0.05, which is close to the theoretical value of alpha(l), = 0.16. The crossover chain lengths are i(c) approximate to 50 for n-BMA and i(c) approximate to 70 for t-BMA. The minor differences in composite-model parameter values of n-BMA and t-BMA are assigned to differences in chain mobility.